Day 8 :: Minimum Stack

src/main/java/com/vitalish/leetcode/stack/MinStack.java

@@ -0,0 +1,44 @@
+package com.vitalish.leetcode.stack;
+
+/**
+ * Minimum Stack
+ *
+ * @see https://neetcode.io/problems/minimum-stack
+ */
+public class MinStack {
+
+    private Node<Integer> top;
+    private Node<Integer> min;
+
+    public MinStack() {
+
+    }
+
+    public void push(int val) {
+        min = new Node<>(Math.min(val, min != null ? min.value : val), min);
+        top = new Node<>(val, top);
+    }
+
+    public void pop() {
+        min = min.next;
+        top = top.next;
+    }
+
+    public int top() {
+        return top.value;
+    }
+
+    public int getMin() {
+        return min.value;
+    }
+
+    private record Node<Integer>(
+            Integer value,
+            Node<Integer> next
+    ) {
+        private Node(Integer value) {
+            this(value, null);
+        }
+    }
+
+}

src/main/java/com/vitalish/leetcode/stack/MinStackWithInternalStack.java

@@ -0,0 +1,33 @@
+package com.vitalish.leetcode.stack;
+
+import java.util.Stack;
+
+public class MinStackWithInternalStack {
+
+    private Stack<Integer> top;
+    private Stack<Integer> min;
+
+    public MinStackWithInternalStack() {
+        top = new Stack<>();
+        min = new Stack<>();
+    }
+
+    public void push(int val) {
+        top.push(val);
+        min.push(Math.min(min.isEmpty() ? val : min.peek(), val));
+    }
+
+    public void pop() {
+        top.pop();
+        min.pop();
+    }
+
+    public int top() {
+        return top.peek();
+    }
+
+    public int getMin() {
+        return min.peek();
+    }
+
+}

src/test/java/com/vitalish/leetcode/stack/MinStackTest.java

@@ -0,0 +1,35 @@
+package com.vitalish.leetcode.stack;
+
+import org.junit.jupiter.api.Assertions;
+import org.junit.jupiter.api.Test;
+
+class MinStackTest {
+
+    @Test
+    void checksMinStack() {
+        MinStack minStack = new MinStack();
+        minStack.push(1);
+        minStack.push(2);
+        minStack.push(0);
+
+        Assertions.assertEquals(0, minStack.getMin()); // return 0
+
+        minStack.pop();
+        Assertions.assertEquals(2, minStack.top());    // return 2
+        Assertions.assertEquals(1, minStack.getMin()); // return 1
+    }
+
+    @Test
+    void checksMinStackWithInternalStack() {
+        MinStackWithInternalStack minStack = new MinStackWithInternalStack();
+        minStack.push(1);
+        minStack.push(2);
+        minStack.push(0);
+
+        Assertions.assertEquals(0, minStack.getMin()); // return 0
+
+        minStack.pop();
+        Assertions.assertEquals(2, minStack.top());    // return 2
+        Assertions.assertEquals(1, minStack.getMin()); // return 1
+    }
+}